1. Field of the Invention
The present invention relates to a process for obtaining radiographic images using a silver halide photographic sensitive material and to an improvement in photographic sensitive materials for such a process. In more detail, the present invention relates to a process for obtaining radiographic images which comprises exposing a silver halide photographic sensitive material to ionizing radiation while in contact with a fluorescent intensifying screen and carrying out photographic processing, and to silver halide photographic sensitive materials for such a process.
2. Description of the Prior Art
For X-ray recording, usually, an intensifying screen or a fluorescent plate are used together with a silver halide photographic film in order to enhance the X-ray sensitivity. Since excess exposure to X-rays is harmful to the human body, devices which enhance X-ray sensitivity so as to obtain X-ray photographic images at lower X-ray closes are often used. For example it has been attempted not only to increase the sensitivity of silver halide photographic emulsions but also to develop systems using an X-ray image intensifier or systems using a solid state light amplifier. However, in any such case, the X-ray images are finally recorded on gelatino silver halide photograhic material as a fluorescent image.
As fluorescent substances hitherto used for such purposes, there are those which emit blue fluorescent light, such as barium sulfate activated by strontium, barium sulfate activated by lead, barium sulfate activated by silver, calcium tungstate (CaWO.sub.4) activated by lead, zinc sulfide activated by silver and barium phosphate (Ba.sub.3 (PO.sub.4).sub.2) activated by europium, etc. For medical treatments, an intensifying fluorescent screen used together with a silver halide radiographic material generally contains calcium tungstate or barium sulfate activated by lead as the fluorescent substance. Recently, studies on fluorescent intensifying screens having a high emission energy intensity have been performed because of increased requirements on X-ray photography for medical treatment. Particularly, it is known from reports of Lockheed Aircraft Corp. that rare earth oxysulfide and oxyhalide fluorescent substances activated by another rare earth element have a high emission energy intensity.
On the other hand, it is preferred that X-ray sensitive materials -- direct X-ray sensitive materials and indirect X-ray sensitive materials- be easily processable, for example, at development or fixing. It is particularly preferred to be able to process them in a light room. This type of X-ray photographic material has been processed under a safe light using, for example, a No. 7 safe light filter produced by Fuji Photo Film Co., Ltd.
The spectral transmission curve of the Fuji Photo Film No. 7 Filter is shown in FIG. 1. It is preferred that radiographic silver halide sensitive materials have a high sensitivity to green fluorescent light emitted upon excitation by the above described fluorescent substance but also have a low sensitivity to safe light for ease of processing.
Recently, the processing time for the development-fixing step of sensitive materials composed of silver halide photograhic emulsions has been shortened to 60 seconds to 120 seconds. In such photographic processing, it is difficult to remove sensitizing dyes included in the photographic emulsions from the sensitive material, and, consequently, color from residual dyes is easily caused.
For the spectral sensitization in the green wavelength range, merocyanine dyes, hemicyanine dyes and trinuclear cyanine dyes have often been used. However, such dyes are unsuitable for sensitizing a specific narrow wavelength range because the spectral sensitivity distribution brought about by using them is too broad. Further, it is difficult to obtain a high sensitivity. Such are particularly disadvantageous for the spectral sensitization of high speed silver iodobromide photographic emulsions since the sensitization ratio is low. Further, it is difficult to find supersensitizing agents for such sensitizing dyes. J-agglomeration type cyanine dyes are advantageously used for such purpose, as is well known. For example, there are known techniques of using imidazolocarbocyanine dyes; see U.S. Pat. Nos. 2,701,198, 2,945,763, 2,973,264, 3,173,791, 3,364,031, 3,397,060 and 3,506,443, Japanese Patent Publication No. 4936/1968 and German Patent Applications (OLS) Nos. 1,944,751, 2,011,879, 2,018,687 and 2,030,326; techniques of using imidazooxacarbocyanine dyes; see Japanese Patent Publication No. 14030/1969; and techniques of using pseudocyanine dyes; see German Patent Application (OLS) No. 1,936,262 and French Pat. No. 1,488,057.
Although some of such known cyanine dyes give a high spectral sensitivity, they are not satisfactory because they do not correlate with the spectrum of light emitted from the green fluorescent substance having a high emission sensitivity since the sensitized wavelength is too long and they cause intolerable residual color on the sensitive material after photographic processing.